1. Field of the Invention
This invention relates to a bonded magnet-forming composition which can be compacted into a bonded magnet featuring mechanical strength and a magnet roller formed from the composition.
2. Prior Art
A jumping development system is well known in the field of electrophotographic machines such as copiers and printers and electrostatic recording machines. One typical developing roller for developing an electrostatic latent image on a latent image-bearing body typically in the form of a photoconductor drum into a visual image includes a rotating sleeve and a static magnet roller disposed therein and formed from a bonded magnet. The magnet roller has such magnetic properties such that a magnetic developing agent or toner borne on the sleeve surface may be forced to jump to the latent image-bearing body. Through this jumping development, the toner is supplied to the surface of the latent image-bearing body for visualizing the electrostatic latent image.
In general, the magnet roller is prepared from a bonded magnet-forming composition comprising a binder of thermoplastic resin such as nylon and polypropylene and magnetic powder such as ferrite. The bonded magnet-forming composition commonly available in pellet form is injection molded or extrusion molded in a mold across which a magnetic field is applied whereby the composition is shaped into a roller while magnetized to desired magnetic properties. The roller is generally provided with a shaft for supporting the roller. The shaft may take various forms, for example, an elongated shaft axially extending through the roller and a pair of shaft stubs projecting from the roller at axially opposite ends. The shaft stubs at axially opposite ends may be separate members or integral members formed integral with the roller body from a bonded magnet-forming composition. The shaft of the magnet roller is sometimes provided with a gear which is used to drive the roller for rotation. Where the shaft stubs are formed integral with the roller body from a bonded magnet-forming composition as mentioned just above, the gear may also be formed integral with the shaft stub and roller body from the bonded magnet-forming composition (see Japanese Utility Model Application Kokai No. 114809/1986).
Magnet rollers formed from prior art bonded magnet-forming compositions using general-purpose resins such as polypropylene as a binder, however, are not always sufficient in mechanical strength. For example, it is difficult to fill polypropylene with a large amount of ferrite for the following reasons. (1) Since molten polypropylene has a high viscosity, it is difficult to uniformly disperse ferrite therein upon kneading. (2) Since molten polypropylene has low strength, polypropylene with a high loading of ferrite is difficult to pelletize. When polypropylene with a high loading of ferrite is milled, extruded into strands and cut into pellets, strands are torn into pieces. Since high loading of polypropylene with ferrite is difficult for this and other reason, the filled polypropylene cannot have sufficient stiffness. Additionally, polypropylene is a nonpolar resin which cannot effectively wet ferrite. This is another reason why sufficient strength is not obtained.
Magnet rollers formed from prior art bonded magnet-forming compositions using general-purpose resins, typically polypropylene of general grade as a binder suffer from problems including low mechanical strength, difficulty to form reduced diameter rollers, and a limited range of application. Where a magnet roller is used as a developing roller and cleaning roller in electrophotographic equipment, for example, the bonded magnet-forming composition should have mechanical strength as represented by a flexural modulus of about 650 kgf/mm.sup.2 and a flexural strength of about 4.5 kgf/mm.sup.2. Such mechanical strength can be accomplished by few of prior art bonded magnet-forming compositions using general-purpose resins such as general grade polypropylene as a binder. Where the shaft is molded integral with the roller body from a bonded magnet-forming composition as previously mentioned, it is difficult to mold the shaft so as to have satisfactory strength and especially to mold an integral gear having satisfactory strength together with the shaft.